Project Summary Sedentary behavior (i.e., sitting) has emerged as an important risk factor for type 2 diabetes, cardiovascular disease, some cancers, and mortality. Working adults with desk-bound occupations accumulate large volumes of daily sedentary time. Sit-stand workstations are now the fastest growing employee benefit, yet evidence-based interventions are not being implemented to support their use. Our team has completed the largest and most definitive efficacy trial on the impact of an intervention to support use of sit-stand workstations and reduce sedentary behavior in the workplace. Stand & Move at Work (SMW) is a multicomponent, social-ecological behavioral intervention tested in 24 industry, government, and academic worksites (N=630 workers). Intervention effects included: (a) reductions in sedentary time at 12 months (~60 min/8 h workday) that were retained at 24- month follow-up (~30 min/8 h workday); (b) reduced body weight and improved chronic disease risk factors among those with high baseline risk; and (c) reduced musculoskeletal pain. We are now proposing a new trial that tests the role of expert-based facilitation to enhance effectiveness and implementation of the SMW intervention. We use the Integrated - Promoting Action on Research Implementation in Health Services (iPARIHS) framework to inform our new trial through: (a) organizing our implementation outcomes from our efficacy trial; (b) conducting industry-based discovery interviews; and (c) piloting our enhanced implementation strategy in new worksites. We propose a 2-arm group-randomized hybrid effectiveness-implementation (type 2) trial to test the effectiveness of SMW for reducing sedentary time in the workplace, and to test an implementation strategy (i.e., expert facilitation) for improving implementation fidelity. Worksites (N=24) will be observed over 3 months of sit-stand workstation use only, and will then be randomized to 12 months of either: (a) SMW (web- delivered); or (b) SMW+ (web-delivered + expert facilitation). Our dual primary outcomes will be reductions in objectively-measured sedentary time (effectiveness) and intervention fidelity (implementation) over 12 months. Because facilitation increases intervention cost, we will also assess incremental cost benefit of our interventions (secondary aim). Finally, we will explore fidelity as a driver of effectiveness, examine within-worksite differences in sedentary time pre- and post- implementation, and measure organizational sustainability of effectiveness and implementation at 24 months. The potential health benefits of sit-stand workstations and associated worksite health promotion programs will not be realized in the workforce at large until we test the most effective and efficient way to implement evidence-based interventions. This project is among the first initiatives to address this growing trend in worksite health, and will answer important questions related to effectiveness, implementation, and cost benefit. Optimal strategies for delivering the SMW intervention will be identified and new knowledge will be generated on how facilitation can enhance implementation fidelity of workplace health initiatives, both of which will increase the public health impact of evidence-based interventions.